Automatic fading control circuit



March 21, 1939. F. KLUTKE 2,151,120

AUTOMATIC FADING CONTROL CIRCUIT Filed May 15, 1936 fignafrgn Ogrecfor IQ 4.5. Agfwork IQ a iiq has SOQLQ i Q L/m/'f/hg Res/sfor INVENTOR FRITZ KL U7'KE- BY x/ ATTORNEY Patented Mar. 21, 1939 UNITED STATES PATENT OFFICE gemeine Elektricitatz many Application May 15,

Gesellschaft, Berlin, Ger- 1936, Serial No. 79.,878

In Germany October 11, 1935 3 Claims.

To overcome fading phenomena in receivers, high-frequency amplifiers are now in use in which the degree of amplification is made dependent upon the amplitude of the wave to be received,

5 in a manner known as such. In this method of fading compensation, a sufficiently high amplification of high-frequency signals must be presupposed in order that reception of sufiicient volume be obtainable even at an infinitely small ,0 receiving amplitude. Therefore, this method is suited only for large receivers in which several high-frequency amplifier stages are available. In case of medium and small receivers fading cannot be compensated by hitherto known means.

It is the object of the present invention to provide a method by which fading compensation is made possible in medium and smaller receivers. The invention is based upon the following reason ing. If a screen grid tube is used as an audion or grid leak detector, and if a positive potential is applied to said screen grid that is higher than that of the anode, the so-called dynatron effect will be obtained (see treatise by A. W. Hull in Jahrbuch fuer drahtlose Telegraphie und Telephonie 14th volume, 1919). The tube then represents a negative resistance which produces a damping reduction in the plate circuit of the audion stage. The extent of this effect depends at proper adjustment of the working point of the tube, upon the potential difference between anode and screen grid. This potential difference is produced by the voltage drop caused by the plate current at an external resistor. Since in grid leak detector circuits, the average control grid potential will be further displaced into the region of negative biasing potential at an increase in the receiving amplitude, whereby the plate current decreases, the dynatron eifect likewise becomes less efiective at an increase in the received signal amplitude.

If, however, the signal amplitude decreases owing to fading phenomena, the plate current increases, and the damping reduction of the audio-frequency part of the receiver increases.

A circuit in accordance with the invention is shown in the drawing. In the latter there is shown a screen grid tube l which is provided with a cathode 5, a signal grid 4, a screen grid 3 and a plate 2. Between the signal grid t and grounded cathode 5 is connected the resonant input circuit 6 upon which the signals are impressed. The network 1 which includes the condenser and Usual grid leak resistor permits the tube I to function as the usual grid leak detector. The plate 2 is connected to a source of positive potential A through a pair of resistors 11 and 12 arranged in series, and the screen grid 3 is connected to the junction of resistor in and T2. The plate side of resistor 11 is connected to the signal grid of the audio amplifier 8 by means of a coupling condenser I0, and the signal grid of the audio amplifier 8 is connected to a source of grid bias C in the usual manner. At the resistor T1, the plate current produces the voltage drop which causes the dynatron effect. The resistor 12 serves as a current limiting resistor. The dimensions of the resistors 11 and T2 are so chosen in accordance with the tube employed, that a favorable working point will be obtained. Therefore, a circuit according to the invention has the great advantage that the receiving amplitude of the audiofrequency part of a receiver will be automatically controlled by the dynatron circuit of the audion tube in such a manner that it counteracts the fading phenomena.

What is claimed is:

1. In combination with a source of signal modulated carrier waves and an audio amplifier, a detector tube which includes at least a cathode, a signal grid, an output plate electrode and an additional cold electrode disposed between the output electrode and signal grid, means including a current source and pair of impedances arranged in series for establishing said output electrode at a sufficiently less positive potential than said cold electrode to impart a dynatron characteristic to said detector tube, means for coupling the input electrodes of the audio amplifier to the output electrode and cathode of said detector across said pair of impedances, and means coupling said carrier wave source to the cathode and signal grid of the detector tube.

2. In combination with a source of signal modulated carrier waves and an audio amplifier, a detector tube which includes at least a cathode, a signal grid, an output electrode and an additional cold electrode disposed between the output electrode and signal grid, means for establishing said output electrode at a sufficiently less positive potential than said cold electrode to impart a dynatron characteristic to said detector tube, means for coupling the input electrodes of the audio amplifier to the output electrode and cathode of said detector, means coupling said carrier wave source to the cathode and signal grid of the detector tube, and said establishing means including a pair of resistors arranged in series with a current source, one of said resistors being connected between the said output electrode and said cold electrode, and the other resistor being of the current limiting type and said input electrodes being connected across both of said resistors.

3. In combination with a source of signal modulated carrier waves and an audio utilization network, a detector tube of the type which includes a cathode, a signal input electrode and an output electrode, and an additional electrode which is at a positive potential with respect to the output electrode and imparts a negative resistance'characteristic to the output electrode circuit of the detector tube, a signal input circuit coupling the carrier Wave source solely to the input electrode and cathode of the detector tube, and means coupling solely the said output electrode and cathode of the detector tube to the audio utilization network; and said detector input circuit including means, responsive to an increase in carrier Wave amplitude, for reducing the magnitude of said negative resistance.

FRITZ KLUIKE. l 

